1. Field of the Invention
The present invention relates to a signal supply circuit for supplying a signal to a plurality of synchronizing circuits.
2. Description of the Background Art
A. Background Art
There are two major types of systems for supplying a signal to synchronizing circuits; that is, the trunk type system and the tree type system. In the following, conventional techniques using these two systems will be described with respect to both structure and associated problems.
(A-1) Conventional Technique using Trunk Type System:
FIG. 20 is a circuitry diagram showing a conventional trunk type signal supply method. This method is described in "IEEE 1990 CUSTOM INTEGRATED CIRCUITS CONFERENCE PROCEEDING," 16.4.1 (FIG. 1). A signal input driver circuit i includes an input terminal and an output terminal which is connected to a trunk wire 2. A synchronizing circuit group 300a is connected to the trunk wire 2 with a certain distance. The synchronizing circuit group 300a consists of thirty-two synchronizing circuits 301 to 332 to which a signal is to be supplied. Although in reality reference numerals are to be assigned to the synchronizing circuits as 301 to 332 from the left hand-side to the right hand-side, for clarity of illustration, not all of the reference numerals are shown in FIG. 1. The synchronizing circuits 301 to 332 each have an input terminal and an output terminal. The input terminals of the synchronizing circuits 301 to 332 are each connected to the circuits 301 to 332 are each connected to the trunk wire 2.
In the circuit which has such a structure as above, a signal supplied to the signal input driver 1 is transferred on the trunk wire 2 to the plurality of the synchronizing circuits 301 to 332, during which a wire-induced delay is created within the trunk wire 2. Hence, the signal arrives at the synchronizing circuits with a greater delay as the synchronizing circuits are farther from the driver 1. Differences in signal transmission time among the synchronizing circuits 301 to 332 is known as skews.
Basically, a skew developed within the trunk wire 2 is strongly influenced by a wire resistance R, a wire capacitance C1 of the trunk wire 2 and an input capacitance Ci of the synchronizing circuits as a whole. The skew, which is desired to be reduced, is expressed as a function of R.times.(C1+Ci). Since the wire resistance R is in inverse proportion to the width of the trunk wire 2, in the trunk type system, the trunk wire 2 is formed to have a large width so that the wire resistance R is suppressed small to reduce the skews.
On the other hand, the wire capacitance C1 increases in proportion to the width of the trunk wire 2. Hence, effective reduction of the skews by the wire resistance R is possible only when the input capacitance Ci is sufficiently larger than the wire capacitance C1. Even in such a case, however, as the width of the trunk wire 2 is formed larger and larger, the skews would amount as much as a time constant R.times.C1 of the wire. Once increased to nearly R.times.C1, the skew will not become smaller from that value. Further, since the wire resistance R and the wire capacitance C1 are each in proportion to the length of the trunk wire 2, the skews increase as the trunk wire 2 becomes longer.
(A-2) Conventional Technique using Tree Type System:
FIG. 21 is a circuitry diagram showing a conventional tree type signal supply method. This method is known as a driving method for supplying a signal to a plurality of synchronizing circuits which are arranged on the same straight line. An output terminal of the signal input driver circuit 1 is connected to a tree wire 200a. Input terminals of the synchronizing circuits 301 to 332 of the synchronizing circuit group 300a are also respectively connected to the tree wire 200a.
In the tree type system, the wire is disposed so that the paths (i.e., wires) from the signal input driver circuit 1 to the respective synchronizing circuits 301 to 332 have the same distance. Hence, unlike the trunk type system described at the section (A-1), the respective paths create the same amount of a signal delay, which reduces the skews of the transmitted signal among the synchronizing circuits to zero in an optimal case.
However, as shown in FIG. 21, a large signal wire area is needed to permit that the wires for supplying a signal to the thirty-two synchronizing circuits 301 to 332 have the same length. For instance, wires 201 to 206 disposed in a direction parallel to the arrangement of the synchronizing circuits 301 to 332 in a 6-layer structure, and hence, an area for forming such wires are necessary. This area for the wires becomes larger as an increased number of the synchronizing circuits are used. In addition, when the synchronizing circuits are not arranged at equal intervals or when the number of the synchronizing circuits cannot be expressed as 2.sup.n or in some other cases, a detour wire must be disposed in a portion of the wire paths in order to adjust a delay time.
(A-3) Associated Problems:
As described above, supply of a signal to a plurality of synchronizing circuits using the trunk and the tree type systems has the following problems.
Problem 1: In the conventional trunk type system where a signal is transferred in a manner described above to the synchronizing circuits which are arranged on the same straight line, a reduction in the value of the skew to nearly zero is physically impossible. If the synchronizing circuits are arranged with a large distance from each other, in particular, even suppression of the skew, let alone a reduction of the skew to nearly zero, is physically impossible.
Particularly in the field of semiconductor, since semiconductor devices become increasingly dense and have a greater capacitance and a larger chip area while more accuracy of a timing signal is demanded due to a higher operation frequency, the trunk type system cannot easily attain a desired timing control among a plurality of synchronizing circuits.
Problem 2: In the conventional tree type system, to deal with a changed condition such as an increase in the number of synchronizing circuits and a necessity of disposing a detour wire for adjustment of a delay time, a large wire area is inevitably and unavoidably necessary for forming wires in a tree-configuration. Especially in the field of semiconductor, this is emerging as an obstruct which prohibits an improvement in the integration of a semiconductor device. One of other problems is an increase in the wire capacitance due to the arrangement in which many wires are located adjacent to each other. Because of these environmental differences with respect to the signal wires, a large skew is often created among synchronizing circuits.